ABSTRACT Adolescents in the juvenile justice system demonstrate very high rates of tobacco, alcohol, and other drug use (ATOD), with rates that are estimated to be three times higher than non-justice-involved youth. Substance- abusing youth ? especially those who are involved in the juvenile justice system ? are at higher risk than nonusers for mental health problems, including depression, conduct problems, personality disorders, suicidal thoughts, attempted suicide, and completed suicide. Additionally, there is evidence of detrimental effects on brain development and stress responsive neurobiological systems from substance abuse. There is growing evidence that interventions based on mindfulness have been an efficacious intervention for a variety of problems, including substance use among adolescents. We propose to develop and test the feasibility of a smart phone application (app) intervention prototype that would help adolescent substance users reduce or quit their substance use. The program, entitled MyMUSE, will apply a mindfulness approach as the guiding framework for the intervention; the program will be evaluated with a high risk group of adolescents that are involved in the juvenile justice system. The proposed program will be the only mobile app specifically for adolescent substance use; there is high level of need for the program both as an adjunct to existing treatments and for use in the juvenile justice system. Our research and development team has extensive experience in working with adolescents that use alcohol, tobacco and other drugs (ATOD), and we have been using mindfulness as the framework for in-person interventions with youth in the juvenile justice system. In the first phase, we will use an iterative development plan to design and develop functionality for the first 4 modules of a MyMUSE prototype app. The research team will partner with InterVision Media who have extensive experience in the development of mobile apps for smartphones, many of which have been commercialized. We will assess navigation and usability with high risk adolescents who are currently reporting substance use. Additional pilot testing will be used to create a prototype version for evaluation. The evaluation study will involve having 60 youth ages 13-17 involved in the juvenile justice system use the app for 6 weeks and complete baseline, 6 weeks and 3-month follow-up. Usability data, program engagement, and changes in attitudes and self-reported substance use will be used to determine the efficacy of the Phase I prototype and evidence for proceeding to Phase II development and evaluation.